The present invention relates to dispersion compensators, and more particularly to a tunable dispersion compensator based on a Bragg grating that allows the dispersion characteristics of a Bragg grating element to be tuned.
Fiber Bragg gratings can be used as effective filter elements in a range of applications in optical communications networks. The dispersion characteristics of a communication link are crucial in determining performance and transmission fidelity. Gratings, particularly chirped gratings can be used effectively for dispersion compensation. The ability to tune the dispersion introduced by a grating element is important in providing an adjustable element for performance optimization.
As optical networks evolve, there is a trend towards the development of xe2x80x9call-opticalxe2x80x9d networks, in which communications signal are dynamically routed in the optical domain. One of the issues that affect the ability of network engineers to increase transmission distances and data rates is signal impairments. One of the important parameters of a link is the dispersion of the system, which can create a serious deleterious effect on the signal quality in a high-Gbit optical transmission systems such as 10 Gbits/s and 40 Gbits/s.
Chirped Bragg gratings can be used to compensate for dispersion in a communications link. FIG. 1 depicts a known optical transmission system 10. Optical data 12, which is generated by a laser transmitter 14, is transmitted along a fiber link 16, that may or may not contain optical amplifiers and other devices (that may in themselves be dispersive) to a receiver 18. During the transmission along the fiber link 16, the optical data 12 will be corrupted to a certain degree by differential group delay experienced by different wavelength components of a given wavelength signal. This group delay xe2x80x9csmears-outxe2x80x9d the data 12 producing poor contrast in the received bit stream 20 at the receiver end. The differential group delay in a signal can be compensated for by the use of an appropriately chirped grating 22. As shown, the light (or received bit stream) 20 from the fiber link 16 is coupled though a circulator 24 to a chirped grating 22. The chirped grating 22 has a bandwidth B and length L. The chirping of the grating 22 results in a strong group delay that can be chosen to be equal to, but opposite to that of the fiber link 16. For example, for a grating length of 10 cm, the differential group delay for a wavelength component reflected from the front of the grating compared to one reflected from the rear of the grating is xcx9c1000 ps. If the grating has a bandwidth of 1 nm, then it""s effective dispersion is 1000 ps/nm. Currently, the grating 22 has to be designed to match a given dispersion induced group delay error in a communications link.
An object of the present invention is to provide a tunable dispersion compensator having a Bragg grating that allows the dispersion characteristics of a Bragg grating element to be tuned.
In accordance with an embodiment of the present invention, an optical waveguide includes an outer cladding disposed about an inner core. A portion of the cladding of the waveguide includes a tapered region. The cladding has a minimum cross-sectional dimension of 0.3 mm. A chirped grating is written in the inner core of the tapered region of the cladding.
In accordance with another embodiment of the present invention, a tunable dispersion compensator includes an optical waveguide having an outer cladding disposed about an inner core. A portion of the cladding has a tapered region. The cladding has a minimum cross-sectional dimension of 0.3 mm. A chirped grating is written in the inner core of the tapered region of the cladding.